1. Field of the Invention
This invention relates to an electrical rotating machine of the revolving-field type, in which magnetic field pole heads and magnetic field pole bodies are fixed together by bolted joints.
2. Description of the Related Art
A synchronous machine is an electrical rotating machine that can realize a large output compared with an induction machine. In recent years, the inverter driving method has been developed so that such synchronous machines can be operated at a desired optional power factor. Especially in regard to synchronous machines for hydroelectric power plants, oil plants, gas plants and the like, there is hence an increasing move toward synchronous machines of larger capacity.
Synchronous machines include two types, one being the revolving-armature type, and the other the revolving-field type. The revolving-field type is the type that a rotor provided with magnetic field poles rotates relative to a stator with armature windings wound thereon. The revolving-armature type outputs an armature current via slip rings, and therefore, involves the wearing of contact portions as a problem. On the other hand, the revolving-field type is free of such a problem, and can simplify the routing of wires. The use of the DC energization method allows the revolving-field type to employ a direct current as its field current so that the magnetomotive force of field windings can be increased even at a low voltage. The revolving-field type which is the subject of the present invention has, therefore, become mainstream in recent years.
In FIGS. 1 and 2, one example of conventionally-known revolving-field type rotors is shown. FIG. 1 is a perspective view of the conventionally-known revolving-field type rotor, and FIG. 2 is a cross-sectional view of the conventionally-known revolving-field type rotor. In the rotor 1 of this example, four magnetic field poles are formed at equal angular intervals around a shaft 1c. Like this rotor 1, at least two or greater even number of magnetic field poles are formed on a revolving-field type rotor. Rotors of a shape that as appreciated from FIG. 2, tip ends of the respective magnetic field poles outwardly project as many as the number of the magnetic field poles are collectively called “salient-pole rotors”.
A shaft body 1b of a square shape in cross-section is formed on a longitudinally central part of the shaft 1c, and on the shaft body 1b, four magnetic field pole bodies 1a that make up shanks of the magnetic field poles are formed. On an outer wall of each magnetic field pole body 1a, a pole shoe 2 that makes up a head of the corresponding magnetic field pole is joined with plural bolts 3. Described specifically, for the bolts 3, a like plural number of through-holes are formed through the pole shoe 2, and threaded hole machining has been applied a like plural number of times to the magnetic field pole body 1a at locations corresponding to the through-holes to form threaded holes. By bringing the bolts 3, which have been inserted in the through-holes, into threaded engagement with the threaded holes, the pole shoe 2 is joined to the magnetic field pole body 1a. Bolted joint portions of each magnetic field pole, where the pole shoe 2 is fixed on the magnetic field pole body 1a, are aligned in at least two parallel rows such that the bolted joint portions are symmetrically located with respect to a central axis of the shaft 1c. Further, a coil 4 is arranged on and around outer periphery of each magnetic field pole body 1a in a space between an outer wall of the shaft body 1b and an inner wall of the pole shoe 2.
Incidentally, centrifugal forces are applied to each bolted joint portion of the rotor 1 during rotation of the rotor 1. As illustrated in FIG. 3, centrifugal forces F1, F2, Fc that are applied to each bolted joint portion of the rotor 1 act in the axial direction of the bolt 3 and, because the acting points of the centrifugal forces F1, F2, Fc deviate from the axial centerline of the bolt 3, moments M1, M2, Mc act on the bolted joint portion. Therefore, on the bolt 3, a pulling stress occurs in the axial direction of the bolt 3, and a bending stress also occurs by the moments M1, M2, Mc.
As shown in FIG. 1, each pole shoe 2 is joined on the corresponding magnetic field pole body 1a with the plural bolts 3. As illustrated in FIG. 4, on an outer side as viewed in a direction perpendicular to an axis of rotation, in other words, on a widthwise outer side of the bolted joint portions of the pole shoe 2, the pole shoe 2 tends to come loose upward from the magnetic field pole body 1a under centrifugal forces, and hence, to result in a phenomenon that the magnetic field pole body 1a and the pole shoe 2 separate from each other at the plane of a joint therebetween. Such a phenomenon becomes more pronounced as the revolution speed required for the electrical rotating machine becomes higher or the overall lengths of the magnetic field pole bodies 1a and pole shoes 2 arranged on the rotor 1 become longer.
When desired to provide a synchronous machine with a large capacity, the rotor 1 may be made longer in the longitudinal direction of the axis of rotation while keeping the same its cross-section perpendicular to the axis of rotation instead of enlarging the cross-section. When the ratio of the axial length to the diameter of the rotor 1 increases, the pole shoe 2 undergoes a greater bending deformation as the distance from its central part increases toward its opposite ends, and therefore, greater bending stresses occur at the bolted joint portions in opposite end portions than at the remaining bolted joint portions.
In a salient-pole rotor, the outer wall of each magnetic field pole is configured such that a magnetic gap becomes larger toward opposite longitudinal ends of the magnetic field pole to make a magnetic flux distribution closer to a sinusoidal waveform. This configuration is effective for reducing harmonics that occur in an induced electromotive force. The centrifugal force to be borne per bolted joint portion, however, becomes greater at both the end portions of the pole shoe 2 as indicated at areas surrounded by solid lines in FIG. 6 compared with at its central part. The pole shoe 2, therefore, undergoes a bending deformation at both the end portions thereof such that it curls up there, leading to a reduction in the magnetic gap to be maintained between the rotor and the stator during operation of the synchronous machine. The occurrence of such a phenomenon also becomes a cause of torque pulsation by harmonics in an induced electromotive force, and therefore, gives not a small influence to the output efficiency.
In general, each bolted joint portion is strong against a pulling force in the axial direction of the bolt 3 but is weak against a force or moment deviating from the axial centerline of the bolt 3, because a bending stress tends to concentrate at the thread groove of the bolt 3. When the pole shoe 2 is sufficiently higher in stiffness than the bolts 3, the centrifugal forces F1, F2, Fc and moments M1, M2, Mc are mostly borne by the pole shoe 2 until the magnetic field pole body 1a and the pole shoe 2 separate from each other at the plane of the joint therebetween. When the revolution speed increases and the separation takes place, however, the loading factor of each bolt 3 increases so that the bolt 3 may fracture.
To provide a revolving-field type synchronous machine with a large capacity, the rotor 1 needs to be enlarged. However, the enlargement of the rotor 1 leads to increases in the centrifugal forces F1, F2, Fc and centrifugal forces F1, F2, Fc, and therefore, the magnetic field pole body 1a and the pole shoe 2 become prone to separation from each other at the plane of the joint therebetween and the bolts are required to bear increased centrifugal forces and moments. For providing a revolving-field type synchronous machine with a large capacity, it is thus important to ensure high strength reliability of bolted joint portions in a salient-pole rotor.
As a measure to meet such a requirement, JP-A-50-155505[U] discloses in FIG. 1 a technology that a rectangular protrusion is formed on a lower wall of each magnetic field pole head with the same height over the entire length of its corresponding magnetic field pole shank and a lower wall of the protrusion is joined to an upper wall of the magnetic field pole shank. According to this technology, the magnetic field pole head can be provided with improved bending stiffness, and therefore, a bending stress which is to act on each bolt can be reduced. In addition, JP-A-50-155505[U] also discloses in FIG. 1 a technology that each magnetic field pole head is beveled at longitudinal opposite end portions thereof to define inclined surfaces. According to this technology, the magnetic field pole head can be reduced in mass at the longitudinal opposite end portions thereof, thereby making it possible to reduce bending stresses and bending moments that are to act on bolts arranged at and around the longitudinal opposite end portions.
On the other hand, JP-A-54-175503 [U] discloses in FIGS. 3 to 5 a technology that a protrusion is formed on a lower wall of each magnetic field pole head on a periphery of bolted joint portions. According to this technology, the protrusion is limited only to the periphery of the bolted joint portions so that the increase in the mass of the magnetic field pole head can be reduced compared with the rotor described in JP-A-50-155505 [U].